Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Disconnect the wiring from the 240v external controls and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler still attempt to fire with no demand?
Note:
The appearance of the white connector may vary depending on the PCB version but the test remains the same.